This could still be done for 1.16, but I'm suggesting this for 1.17 due to it being the version to add lava cauldrons.

Since the existing molten fluids function (or are planned to function) a lot like lava when placed in the world, it may be advisable to also allow them to be placed inside of cauldrons, retaining their lava-like properties and allowing them to be stored physically in the world in a piston-pushable way.

Inspirations adds this feature for 1.16. Probably will keep using that for 1.17, as it supports any fluid, but a lot of this depends on whether forge fixes their API to work with cauldrons.